The "hello world" user context example I demonstrated in class on Tuesday. The diff file attached shows one way you could modify it so that the program context switches back to `main` after `f` finishes.

context.diff

--- context_hello_world.c	2013-10-23 19:47:07.000000000 -0400
+++ context_hello_world1.c	2013-10-23 19:47:13.000000000 -0400
@@ -7,8 +7,11 @@
 
 #define STACKSIZE 4096
 
+ucontext_t main_context;
+
 void f(){
   printf("Hello World\n");
+  setcontext(&main_context);
 }
 
 int main(int argc, char * argv[]){
@@ -40,7 +43,7 @@
   makecontext(&uc, f, 0);
 
   // context switch!
-  setcontext(&uc);
+  swapcontext(&main_context, &uc);
   perror("setcontext"); //setcontext() doesn't return on success
 
   return 0;

context_hello_world.c

#include <ucontext.h>
#include <sys/types.h>
#include <signal.h>
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>

#define STACKSIZE 4096

void f(){
  printf("Hello World\n");
}

int main(int argc, char * argv[]){

  // some initialization
  ucontext_t uc;
  void * stack;

  // initialize the ucontext uc
  getcontext(&uc);

  // allocate some stack space
  stack = malloc(STACKSIZE);

  // uc_stack is just a struct that holds information about a stack,
  // specifically where it is in the heap (i.e. base pointer) and how big it is
  uc.uc_stack.ss_sp = stack;
  uc.uc_stack.ss_size = STACKSIZE;
  uc.uc_stack.ss_flags = 0;

  // we're not blocking any signals in the uc context
  sigemptyset(&(uc.uc_sigmask));

  // when uc terminates, don't switch to another context (i.e. exit the program)
  // i.e. successor context
  uc.uc_link = NULL;

  // modifies the uc context to call the `(void)(*f)` function with no arguments
  makecontext(&uc, f, 0);

  // context switch!
  setcontext(&uc);
  perror("setcontext"); //setcontext() doesn't return on success

  return 0;
}